Alternative fuel gasses, such as natural gas and hydrogen, are promising alternatives to the traditional petroleum-based energy sources used in automotive vehicles. They are cleaner burning than petroleum-based gasoline and diesel, and are therefore better for the environment. Two prevailing technologies exist for storing such fuel gasses aboard a vehicle—in a compressed state or on a gas storage material. Compressed natural gas, for example, is stored at high pressure to less than 1% of the volume it would normally occupy at standard temperature and pressure. Natural gas can also be stored on a storage material (ANG storage material) in an adsorbed state. The allure of such ANG storage materials is that they can reversibly adsorb natural gas at an energy density comparable to compressed natural gas but at a much lower tank pressure.
Hydrogen gas, like natural gas, can also be stored in a compressed state or on a hydrogen storage material. Storing hydrogen gas on a hydrogen storage material has similar thermodynamics to storing natural gas on an ANG storage material even though hydrogen uptake is chemical in nature—hydrogen is stored as a hydride—as opposed to adsorptive. Hydrogen gas, for instance, can be reversibly charged and released from a hydrogen storage material such as, for example, a complex metal hydride including various known alanates, borohydrides, and amides. Some specific complex metal hydrides include sodium alanate (NaAlH4), lithium alanate (LiAlH4), lithium borohydride (LiBH4) with or without MgH2, calcium borohydride (CaBH4) with or without MgH2, and lithium amide (LiNH2). MOFs and PPNs may also be used to store hydrogen gas. There are, of course, many other hydrogen storage materials that are commercially available.
While natural gas and hydrogen can be stored on their respective ANG and hydrogen storage materials at a lower pressure, compared to being stored in a compressed state, the time needed to fill a fuel gas tank that houses the storage material can be extensive since the natural gas adsorption and hydrogen uptake processes are exothermic and have a rate-limiting effect on further adsorption/uptake. Charging enough natural gas or hydrogen into a storage tank in direct contact with an appropriate gas storage material to provide a reasonable driving distance for a vehicle can take many hours to accomplish. Such long filling times may not always be acceptable or convenient for vehicle power applications.